Project Summary/Abstract Stroke is the leading cause of long-term disability is the U.S. Individuals with hemiparesis due to stroke often have difficulty bearing weight on the paretic lower extremity and transferring weight from one leg to the other. Impaired weight transfer and limb loading contribute to lateral instability and are associated with decreased walking speed and increased risk of falling. Consequently, restoring limb loading ability is an important goal for rehabilitation post-stroke. Despite considerable rehabilitation efforts aimed at enhancing paretic limb loading, their effectiveness on improving neuromotor and functional outcomes remains limited possibly due to poorly understood limb loading mechanisms and the reluctance to use the paretic limb. The coordination of neuromuscular actions to regulate loading force during weight acceptance is an important component of functional limb loading. . Because altered neuromuscular control is common in persons with stroke, it is possible that these abnormalities may impair limb loading ability. The long term objective of this project is to develop a mechanism-based framework for designing and testing the effectiveness of novel rehabilitation interventions to enhance lower limb weight transfer and limb loading to improve balance and mobility. This project aims to (1) identify the neuromuscular and biomechanical abnormalities in limb loading responses in individuals post-stroke, (2) determine the underlying mechanisms responsible for the deficits in limb loading, and (3) test the short-term effectiveness of a 6-week perturbation-induced limb load training program on improving limb loading responses and mobility function. We propose to apply a sudden unilateral lowering of support surface to induce lateral weight transfer that forces limb loading. Kinetic, kinematic, and lower extremity muscle activation patterns will be recorded. We expect that, compared to healthy controls, individuals with stroke will show increased muscle co-activation of the knee musculature with decreased knee flexion and torque production, and irregular impact force regulation during loading that will disrupt weight transfer and loading of the paretic limb. Furthermore, we hypothesize that compared to a conventional clinical weight-shift rehabilitation training program, the imposed limb loading group will show greater improvements during voluntary stepping and walking following training. Specifically, we expect the knee muscle co-activation duration will be reduced, with increased knee joint torque, and the paretic single stance/double support time will increase, reflecting improved paretic limb loading ability during gait following training.